[article] Create a lit cheat sheet

packages/lit-dev-content/samples/articles/lit-cheat-sheet/add-styles/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/add-styles/my-element.ts

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+import { html, LitElement, css } from 'lit';
+import { customElement } from 'lit/decorators.js';
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  render() {
+    return html`<p>I'm blue</p><div>I'm red</div>`;
+  }
+
+  static styles = css`
+    p {
+      color: blue;
+    }
+    div {
+      color: red;
+    }
+  `;
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/add-styles/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "100px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/bind-tag-name/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/bind-tag-name/input-or-textfield.ts

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+import { html, LitElement } from 'lit';
+import { customElement, property } from 'lit/decorators.js';
+import { html as staticHTML, StaticValue } from 'lit/static-html.js';
+import { live } from 'lit/directives/live.js';
+
+@customElement('input-or-textfield')
+export class MyElement extends LitElement {
+  // attribute is false because this is a value that can't be serialized to an
+  // HTML attribute
+  @property({ attribute: false }) tagLiteral: StaticValue|null = null;
+  @property() value = '';
+
+  render() {
+    return html`
+      ${
+        // NOTE: the live() directive prevents setting the .value property if
+        // the live value of the input / textfield already matches this.value.
+        // This is important since static html templates should not be thrashed
+        // due to performance concerns.
+        staticHTML`
+          <${this.tagLiteral}
+            @input=${this.#onInput}
+            .value=${live(this.value)}></${this.tagLiteral}>
+        `
+      }
+      <div>
+        The value of the input is: ${this.value}
+      </div>
+    `;
+  }
+
+  #onInput(e: InputEvent) {
+    this.value = (e.target as (HTMLInputElement | HTMLTextAreaElement)).value;
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/bind-tag-name/my-element.ts

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+import { css, html, LitElement } from 'lit';
+import { customElement, state } from 'lit/decorators.js';
+import { literal } from 'lit/static-html.js';
+import './input-or-textfield.js';
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  @state() tagLiteral = literal`input`;
+  render() {
+    return html`
+      <!-- /* playground-fold */ -->
+      <fieldset>
+        <legend>Choose a tag to render:</legend>
+        <div>
+          <label>
+            <input
+                type="radio"
+                name="selection"
+                @change=${this.#onChange}
+                value="input"
+                checked>
+            input
+          </label>
+        </div>
+        <div>
+          <label>
+            <input
+                type="radio"
+                name="selection"
+                @change=${this.#onChange}
+                value="textarea">
+            textarea
+          </label>
+        </div>
+      </fieldset>
+      <!-- /* playground-fold-end */ -->
+      <input-or-textfield
+          value="this is the default value"
+          .tagLiteral=${this.tagLiteral}>
+      </input-or-textfield>
+    `;
+  }
+
+  #onChange(e: InputEvent) {
+    const target = e.target as HTMLInputElement;
+    this.tagLiteral = target.value === 'input' ? literal`input` : literal`textarea`;
+  }
+
+  static styles = css`/* playground-fold */:host { font-family: sans-serif; } :host > * { margin-block: .5em; }/* playground-fold-end */`;
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/bind-tag-name/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "input-or-textfield.ts": {},
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "175px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/classes/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/classes/my-element.ts

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+import { html, LitElement, css } from 'lit';
+import { customElement, state } from 'lit/decorators.js';
+import { classMap } from 'lit/directives/class-map.js';
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  @state() counter = 0
+
+  firstUpdated() {
+    setInterval(() =>  this.counter += 1 , 1000);
+  }
+
+  render() {
+    const classes = {
+      red: this.counter % 2 === 0,
+      blue: this.counter % 2 === 1
+    };
+    return html`<p class=${classMap(classes)}>Hello!</p>`;
+  }
+
+  static styles = css`
+    .red {
+      color: red;
+    }
+    .blue {
+      color: blue;
+    }
+  `;
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/classes/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "100px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/conditionals/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/conditionals/my-element.ts

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+import { html, LitElement } from 'lit';
+import { customElement, state } from 'lit/decorators.js';
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  @state() private someBoolean = false;
+
+  render() {
+    let someText = html`<p>Some text</p>`;
+
+    if (this.someBoolean) {
+      someText = html`<p>Some other text</p>`;
+    }
+
+    return html`
+      <button
+          @click=${() => {this.someBoolean = !this.someBoolean}}>
+        Toggle template
+      </button>
+      <div>This is an inline ternary conditional</div>
+      ${this.someBoolean ? html`<p>Some other text</p>` : html`<p>Some text</p>`}
+      <div>This is a variable conditional</div>
+      ${someText}
+    `;
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/conditionals/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "200px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/constructable-stylesheets/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/constructable-stylesheets/my-element.ts

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+import { html, LitElement } from 'lit';
+import { customElement } from 'lit/decorators.js';
+import { trustedStyles, type CSSStyleSheet } from './trusted-stringified-css-source.js';
+
+// Use constructable stylesheets on TRUSTED CSS strings to use them in a LitElement
+const styles = new CSSStyleSheet();
+// this type assertion is needed for the older version of TS like that the lit.dev website uses
+(styles as unknown as CSSStyleSheet).replace(trustedStyles);
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  static styles = styles;
+  render() {
+    return html`
+      <div>
+        This should be red!
+      </div>
+    `;
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/constructable-stylesheets/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "trusted-stringified-css-source.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "100px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/constructable-stylesheets/trusted-stringified-css-source.ts

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+export const trustedStyles = `
+  div {
+    color: red;
+  }
+`;
+
+// This may be needed for some older versions of TS
+export type CSSStyleSheet = typeof globalThis['CSSStyleSheet'] & {
+  replaceSync(cssText: string): void;
+  replace(cssText: string): void;
+};

packages/lit-dev-content/samples/articles/lit-cheat-sheet/css-shadow-parts/index.html

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+<script type="module" src="./my-element.js"></script>
+<style>
+  .styled my-element::part(paragraph) {
+    color: yellow;
+    border-color: white;
+    padding: 8px;
+    margin: 2px;
+  }
+
+  .styled {
+    background-color: black;
+  }
+
+  div {
+    padding: 4px;
+  }
+</style>
+
+<div class="styled">
+  <my-element></my-element>
+</div>
+<div>
+  <my-element></my-element>
+</div>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/css-shadow-parts/my-element.ts

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+import { html, LitElement, css } from 'lit';
+import { customElement } from 'lit/decorators.js';
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  static styles = css`
+    p {
+      color: blue;
+      border: 1px solid black;
+      padding: 4px;
+      margin-block: 4px;
+    }
+  `;
+
+  render() {
+    return html`<p part="paragraph">This is in a shadow root!</p>`;
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/css-shadow-parts/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "120px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/custom-attribute-converter/index.html

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+<script type="module" src="./my-element.js"></script>
+
+<my-element array='1,"2",3,4,"5"'></my-element>

packages/lit-dev-content/samples/articles/lit-cheat-sheet/custom-attribute-converter/my-element.ts

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+import { html, LitElement } from 'lit';
+import { customElement, property, state } from 'lit/decorators.js';import {ComplexAttributeConverter} from 'lit';
+
+/**
+ * Bidirectionally converts an array from an attribute to a property of the
+ * following format:
+ *
+ * array-attribute='1, "2", 3' to [1, '2', 3]
+ */
+export const arrayConverter: ComplexAttributeConverter<Array<unknown>> = {
+  toAttribute: (array: Array<unknown>) => {
+    return JSON.stringify(array).substring(1, JSON.stringify(array).length - 1);
+  },
+  fromAttribute: (value: string) => {
+    try {
+      return JSON.parse(`[${value}]`);
+    } catch {
+      return [];
+    }
+  }
+};
+
+@customElement('my-element')
+export class MyElement extends LitElement {
+  @property({ converter: arrayConverter, reflect: true })
+  array: Array<number|string> = [];
+
+  render() {
+    return this.array.map((item) =>
+      html`<div>${typeof item}: ${item}</div>`
+    );
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/custom-attribute-converter/project.json

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+{
+  "extends": "/samples/v3-base.json",
+  "files": {
+    "my-element.ts": {},
+    "index.html": {}
+  },
+  "previewHeight": "200px"
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/data-up/game-player.ts

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+import { html, LitElement } from 'lit';
+import { customElement } from 'lit/decorators.js';
+
+export type ScoreEvent = CustomEvent<number>;
+
+@customElement('game-player')
+export class GamePlayer extends LitElement {
+  render() {
+    return html`
+      <button @click=${() => this.handleScore(7)}>Touchdown!</button>
+      <button @click=${() => this.handleScore(3)}>Field goal!</button>
+    `;
+  }
+
+  handleScore(points: number) {
+    this.dispatchEvent(new CustomEvent('score', { detail: points, bubbles: true }));
+  }
+}

packages/lit-dev-content/samples/articles/lit-cheat-sheet/data-up/index.html

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+<script type="module" src="./score-board.js"></script>
+
+<score-board></score-board>